1. Field of the Invention
The invention relates to an internal combustion engine and a method of producing this internal combustion engine. More particularly, the invention relates to an internal combustion engine in which an anodic oxidation coating film is formed on all or a portion of the wall facing a combustion chamber of the internal combustion engine and relates to a method of producing an internal combustion engine having a feature in forming this anodic oxidation coating film.
2. Description of the Related Art
An internal combustion, engine, e.g., a gasoline engine or diesel engine, is composed primarily of an engine block and a cylinder head, and its combustion chamber is defined by the bore surface of the cylinder block, the top surface of the piston installed in this bore, the bottom surface of the cylinder head, and the head surfaces of the intake and exhaust valves that are disposed in the cylinder head. Accompanying the higher outputs being required of internal combustion engines at the present time, it has become crucial to lower their cooling losses. One strategy for lowering this cooling loss is to form a heat-insulating ceramic coating film on the inner wall of the combustion chamber.
However, these ceramics generally have a low thermal conductivity and have a high heat capacity, causing the appearance of a reduced intake efficiency and knocking (abnormal combustion caused by heat being retained within the combustion chamber), and as a consequence at the present time their use as a coating film material on the interior walls of the combustion chamber is not spreading.
In view of the preceding, the heat-insulating coating film formed on the wall of the combustion chamber must certainly be heat resistant and heat insulating and is desirably formed from a material that has a low thermal conductivity and a low heat capacity. Moreover, in addition to this low thermal conductivity and low heat capacity, the coating film is desirably formed from a material that can Withstand the expansion pressure and injection pressure during combustion in the combustion chamber and the repetitive stresses from thermal expansion and thermal shrinkage and is also desirably formed from a material that has a high adherence to the base material of, e.g., the cylinder block and so forth.
When one considers the conventional disclosed technology here, the cylinder head disclosed in Japanese Patent Application Publication No. 2003-113737(JP-A-2003-113737) has a microporous silicon dioxide-type or aluminum oxide-type coating film formed by anodic oxidation on both the bottom surface of the cylinder head and the inner surface of a water jacket that is defined within this cylinder head. Through the disposition in this cylinder head of the microporous coating film on both the bottom surface of the head and the inner surface of the jacket, the surface area of the bottom surface of the head and the jacket inner surface is enlarged by this coating film, and as a result the heat produced in the combustion chamber can be efficiently absorbed into the interior across the coating film and the inwardly absorbed heat can be efficiently discharged across the coating film at the jacket inner surface into the coolant. As a consequence, heating readily occurs through heat absorption while cooling readily occurs through heat release, resulting in a cylinder head for which a temperature increase is suppressed.
The internal combustion engine disclosed in Japanese Patent Application Publication No. 2009-243352 (JP-A-2009-243352) and WO 2009/020206 has a heat-insulating thin film in which bubbles are formed in the interior of a material that has a thermal conductivity lower than that of the base material forming the combustion chamber of the internal combustion engine and that has a heat capacity that is the same as or lower than that of the base material.
The art disclosed in the previously described JP-A-2003-113737, JP-A-2009-243352, and WO 2009/020206 is an art in which a coating film having a low thermal conductivity and a low heat capacity is formed on the inner wall of the combustion chamber of the internal combustion engine, and can provide heat-insulating coating films that exhibit excellent properties as described above.
However, it is not clear whether these coating film structures provide coating films that can withstand the expansion pressure and injection pressure during combustion in the combustion chamber and the repetitive stresses from thermal expansion and thermal shrinkage, or can provide coating films that can relax these pressures and stresses. The inventors found that it would be difficult to say that these coating film structures have an excellent pressure relaxing or stress relaxing behavior. One reason for this is that a coating film produced by anodic oxidation has a microstructure in which the constituent cells have voids in the interior while adjacent cells are almost gaplessly chemically bonded to each other, and as a result it is difficult to set up a satisfactory stress relaxation between these cells.